Catalyst Coating and Process for Production Thereof

ABSTRACT

A process for wet-chemical production of a catalyst coating on an electrically conductive support for electrodes for chloralkali or hydrochloric acid electrolysis with electrocatalytically active components based on noble metal oxides, in which the catalyst coating is produced by; producing a coating solution or dispersion comprising a precursor compound of a noble metal and/or a metal oxide of a noble metal, and a solvent or dispersant, with addition of one or more acids to the coating solution or dispersion, where the molar ratio of the total of the amounts of acid (in mol) present in the coating solution or dispersion to the sum of the amounts of the metals from the metal-containing components present in the coating solution or dispersion is at least 2:1; applying the coating solution or dispersion to the support; substantially freeing the layer applied of solvent or dispersant by drying; and subjecting the dried layer obtained to a thermal treatment to form the catalyst coating.

FIELD OF THE INVENTION

This application claims priority to the DE application 102013202143.7 filed Feb. 8, 2013.

The invention relates to an improved catalyst coating with electrocatalytically active components based on ruthenium oxide and titanium oxide, especially for use in chloralkali or hydrochloric acid electrolysis for production of chlorine. The invention further relates to a production process for the catalyst coating and to a novel electrode.

The present invention specifically describes a process for wet-chemical deposition of mixed oxide layers composed of ruthenium oxide and titanium oxide on a metallic support and the use thereof as electrochemical catalysts in chlorine production by electrolysis.

The invention proceeds from electrodes and electrode coatings known per se, which are typically coated onto an electrically conductive support and comprise catalytically active components including, in particular, electrocatalytically active components based on ruthenium oxide and titanium oxide.

BACKGROUND OF THE INVENTION

Chlorine is produced industrially typically by electrolysis of sodium chloride or hydrochloric acid or by gas phase oxidation of hydrogen chloride (Schmittinger, Chorine, Wiley-VCH 1999, pages 19-27). If electrolysis processes are used, chlorine is produced at the anode. The anode material used is usually titanium as the electrode material, on the surface of which there is an electrochemically active catalyst. The layer on the surface comprising the catalyst is typically also referred to as the coating. The task of the catalyst is to lower the overpotential and prevent evolution of oxygen at the anode (Winnacker-Küchler, Chemische Technik, Prozesse und Produkte [Chemical Technology, Processes and Products], 5th edition, Wiley-VCH 2005, pages 469-470).

In the production of chlorine by electrolysis of hydrochloric acid, graphite anodes are used (Winnacker-Küchler, Chemische Technik, Prozesse und Produkte, 5th edition, Wiley-VCH 2005, page 514). In hydrochloric acid electrolysis, in which a gas diffusion electrode is used, for example, on the cathode side, it is possible to use titanium anodes having noble metal-based catalysts in the coating (Winnacker-Küchler, Chemische Technik, Prozesse und Produkte, 5th edition, Wiley-VCH 2005, page 515).

Electrodes for electrolysis processes are typically based on a metal which is one of the so-called valve metals. Valve metals are understood to mean, for example, the metals titanium, zirconium, tungsten, tantalum and niobium. These act as a diode material for electrical current because of oxide layers on the metal surface.

For use in electrolysis, an electrocatalytically active catalyst comprising a noble metal and/or metal oxide thereof is typically applied on the surface of the valve metals, in which case it is also optionally additionally possible for oxides of the valve metal to be present in the metal oxide (WO 200602843 (ELTECH), BECK, Electrochimica Acta, Vol. 34, No. 6, pages 811-822, 1989)). The oxide-forming noble metal is typically one of the platinum metals, for example iridium, ruthenium, rhodium, palladium, platinum or mixtures thereof. Such electrodes are typically referred to as DSA electrodes, DSA standing for “dimensionally stable anode”.

Disadvantages of these known electrodes when used in halide-containing electrolytes are the overpotential component for the chlorine deposition, which is still high, the tendency of the electrodes to evolve oxygen nonetheless, the high electrolysis voltage and the high demand for costly noble metal for production of the coating. All these factors have an adverse effect on the economic viability of the known electrolysis process using such electrodes.

It is also known from the coatings from the prior art (DE 602005002661 T2) that the noble metals are eluted out of the coating with time under electrolysis conditions; long-term corrosion resistance is accordingly inadequate. The need for corrosion resistance becomes apparent from the fact that the loss of the noble metal-containing coating leads to direct contact of the electrode metal, typically the valve metal, with the electrolyte and to formation of an electrically non-conductive oxide on the surface thereof. For the running of the electrolysis process, this means that no electrochemical operations take place any longer at this surface, which can cause a complete failure of the electrolysis cell with the associated adverse economic consequences.

Moreover, when an electrolyzer with noble metal-containing DSA electrodes is used in chloride-containing solutions for production of chlorine, it is observed that the side reaction of oxygen formation cannot be fully suppressed, as a result of which oxygen is found in the chlorine product gas. The oxygen content means increased purification complexity for the chlorine gas and as a result likewise has adverse effects on the economic viability of the electrolysis. The increased formation of oxygen becomes clearly visible particularly when the chloride concentration in the electrolyte falls, in the case of electrolysis of sodium chloride solutions particularly at a concentration below 200 g/l NaCl, and when the current density is increased, particularly above a current density of 5 kA/m².

Moreover, the sole use of noble metals as catalytic electrode material, because of the high cost thereof and diminishing availability thereof on the global market, likewise impairs the economic viability of known electrodes.

Mixed oxide layers composed of titanium oxide and ruthenium oxide supported on titanium have long been known as stable electrochemical catalysts for chloralkali electrolysis.

These are conventionally produced by means of a thermal decomposition of aqueous or organic ruthenium and titanium salt solutions, which are applied by dipping, brushing or spraying onto a titanium substrate. Each application step is followed by a calcination. In general, several application/calcination steps are needed to achieve the required catalyst loading on the electrode. A disadvantage is that the electrodes thus coated still have a significant overpotential for the electrochemical chlorine production.

A further process for producing mixed oxide layers composed of titanium oxide and ruthenium oxide on a titanium support is sol-gel synthesis. This generally involves application of an organic precursor solution to the titanium. Analogously to the thermal decomposition process, the process requires several complex calcination steps. Likewise disadvantageous in the sol-gel synthesis is the use of very costly organic precursor compounds.

Good mixed oxide formation of titanium oxide and ruthenium oxide is known to be crucial for anodic stability in chlorine electrolysis. Pure ruthenium oxide is corrosion-sensitive with respect to anodic evolution of oxygen, which accompanies evolution of chlorine. It is only the mixed oxide formation of ruthenium oxide with titanium oxide that ensures adequate stability. The effect of mixed oxide formation on electrode stability is described by V. M. Jovanovic, A. Dekanski, P. Despotov, B. Z. Nikolic and R. T. Atanasoski in Journal of Electroanalytical Chemistry 1992, 339, pages 147-165.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved catalyst coating with electrocatalytically active components based on ruthenium oxide and titanium oxide and a process for production thereof, which overcomes the above disadvantages of the coatings known to date and, on application to an electrode, enables a lower overpotential in the evolution of chlorine, for example in chloralkali electrolysis or hydrochloric acid electrolysis. In addition, the catalyst coating should adhere firmly to the base metal and not be attacked chemically or electrochemically. With the catalyst coating, it should be possible to achieve a low electrolysis voltage even at low chloride concentrations.

It is a specific object of the invention to develop a wet-chemical preparation process for mixed oxide layers, which exhibits improved properties over the processes already known.

The above-described object is achieved in accordance with the invention by using a selected catalyst coating based on noble metal oxides, especially ruthenium oxide and/or iridium oxide, and valve metal oxides, especially titanium oxide, in which a solvent or dispersant with a significant excess of acid relative to the catalytic component is used in the production of a coating solution or coating suspension.

DETAILED DECRIPTION OF THE INVENTIION

The invention provides a process for wet-chemical production of a catalyst coating on an electrically conductive support for electrodes for chloralkali or hydrochloric acid electrolysis with electrocatalytically active components based on at least one noble metal oxide and/or noble metal of the noble metals of transition group VIIIa of the Periodic Table of the Elements, especially a noble metal selected from the group of: Ru, Rh, Pd, Os, Ir and Pt, especially ruthenium oxide and/or iridium oxide, and optionally additionally at least one valve metal oxide and/or valve metal, especially with a valve metal selected from the group of Ti, Zr, W, Ta, Nb or oxides thereof, characterized in that the catalyst coating is produced by

-   -   a) producing a coating solution or dispersion at least         comprising a precursor compound of a noble metal and/or a metal         oxide of a noble metal, especially a precursor compound of         ruthenium and/or iridium, and optionally additionally a         precursor compound of a valve metal and/or a metal oxide of a         valve metal and/or a tin compound, preferably a titanium         compound, and a solvent or dispersant, with addition of one or         more acids to the coating solution or dispersion, where the         molar ratio of the total of the amounts of acid (in mol) present         in the coating solution or dispersion to the sum of the amounts         of the metals from the metal-containing components present in         the coating solution or dispersion (i.e. the sum of the metals         present in the coating solution or dispersion in mol) is at         least 2:1, preferably at least 3:1, more preferably at least         4:1,     -   b) applying the coating solution or dispersion to the conductive         support,     -   c) substantially freeing the layer applied of solvent or         dispersant by drying and     -   d) followed by subjecting the dried layer obtained to a thermal         treatment at a temperature of at least 300° C., preferably at         least 400° C., and optionally in the presence of         oxygen-containing gases, to form the catalyst coating.

A significant excess of acid relative to the catalytic component in the coating solution/dispersion is present, especially when the molar ratio of the sum of the amounts of acid (in mol) present in the solution/dispersion to the sum of the amounts of the metals from the metal-containing components present in the coating solution or dispersion (sum of the metals present in mol) is at least 2:1, preferably at least 3:1, more preferably at least 4:1. The acid may be either one or more organic acids or one or more inorganic acids, or a combination of one or more organic and one or more inorganic acids. Preferably, at least one acid is an organic acid. An acid in the context of the invention is present especially when the pK_(a) thereof in aqueous solution is not more than 12, preferably not more than 8 and more preferably not more than 5.

The following equilibrium reaction between an acid HA and its base A—exists in aqueous solution:

HA+H₂O═H₃O⁺+A⁻

The pK_(a) for an acid HA is then determined to be

pK_(a)=−1 g[c(H₃O⁺)×c(A ⁻)/c(HA)/(mol/l)]

where c denotes the concentration of the respective species in mol/l.

It has been found that, surprisingly, when a coating solution or dispersion with a significant acid excess relative to the metallic components present in the coating solution is used, the cell potential required in the electrolysis can be lowered further.

It is a feature of the catalyst coatings prepared by the process according to the invention that they have an exceptional surface morphology compared to those obtainable from known processes.

This exceptional surface morphology apparently increases the active surface area which can be utilized for electrochemical catalysis. Thus, the catalytic activity is improved and the noble metal content can be reduced compared to known catalyst coatings.

A particular embodiment of the novel process is characterized in that the coating solution or dispersion according to step a) additionally comprises a precursor compound and/or a metal and/or a metal oxide of one or more doping elements from the group of: aluminium, antimony, lead, iron, germanium, indium, manganese, molybdenum, niobium, tantalum, titanium, tellurium, vanadium, zinc, tin and zirconium.

A particular embodiment of the novel process involves applying the catalysts to an electrode structure comprising a valve metal, especially a metal from the group of titanium, zirconium, tungsten, tantalum and niobium, as follows: For this purpose, the electrode structure is mechanically pre-cleaned, especially sand-blasted, and can optionally subsequently be etched with an acid, for example a mineral acid such as hydrochloric acid or oxalic acid, for further removal of oxides on the surface.

The surface can be coated using, for example, a coating solution or dispersion comprising a noble metal compound, at least one solvent from the group of: water, C₁-C₆-alcohol, preferably butanol or isopropanol, or a mixture of C₁-C₆-alcohol and water, and an acid and optionally a valve metal compound. Preferred solvents or dispersants are therefore one or more from the group of: water and C₁-C₆-alcohol, more preferably butanol or isopropanol.

The acids used in the coating solution may be one or more organic or inorganic acids or a combination of organic and inorganic acids. Preference is given to a particular process in which the acid used in the coating solution is a combination of organic and inorganic acid, where the molar ratio of the amount of organic acid (in mol) present in the coating solution or dispersion to the amount of mineral acid is 20:80 to 100:0, preferably 50:50 to 100:0, more preferably 80:20 to 100:0.

The organic acids used are preferably water-soluble acids, for example alkanoic acids, alkanedioic acids, mono-, di- and trihaloalkanoic acids, preference being given to using at least one short-chain alkanoic acid such as formic acid, acetic acid or propionic acid, or a short-chain mono-, di- or trihaloalkanoic acid such as chloroacetic acid, dichloroacetic acid, trichloroacetic acid, fluoroacetic acid, difluoroacetic acid or trifluoroacetic acid, or toluenesulphonic acid. The inorganic acids used may be mineral acids, for example hydrohalic acids, preferably hydrochloric acid, sulphuric acid, sulphurous acid, phosphoric acid, phosphorous acid, nitric acid and/or nitrous acid or else phosphonic acids, sulphonic acids, blocked sulphonic acids, or blocked sulphuric and phosphoric acids. Particular preference is given to using mixtures of a mineral acid and an organic acid.

In a particularly preferred process, the acid used in the coating solution in a) is hydrochloric acid and/or a C₁-C₄-carboxylic acid, preferably at least one acid from the group of: hydrochloric acid, formic acid, acetic acid or propionic acid. Very particular preference is given to using a mixture of acetic acid and hydrochloric acid.

The coating solution may additionally, or else instead of one or more of the aforementioned metal compounds, comprise further solids. Preferred solids are polymorphs of carbon or non-noble metal oxides or mixtures thereof.

In that case, the dispersion comprises preferably up to 20% by weight, more preferably to 10% by weight, of further solids, based on the weight of the suspension.

The noble metal precursor compounds used in the novel process are, in particular, solvent-soluble fluorides, chlorides, iodides, bromides, nitrates, phosphates, sulphates, acetates, acetylacetonates or alkoxides of the elements: Ru, Rh, Pd, Os, Ir and Pt, preferably a noble metal chloride, more preferably a ruthenium chloride and/or iridium chloride. Optionally, it is additionally possible to use valve metal precursor compounds, preferably solvent-soluble fluorides, chlorides, iodides, bromides, nitrates, phosphates, sulphates, acetates, acetylacetonates or alkoxides of the elements titanium, zirconium, tungsten, tantalum and niobium, more preferably at least one titanium alkoxide selected from the group of: titanium 2-ethylhexyloxide, titanium ethoxide, titanium isobutoxide, titanium isopropoxide titanium methoxide, titanium n-butoxide, titanium n-propoxide, titanium tert-butoxide, most preferably titanium n-butoxide and/or titanium isopropoxide.

Noble metals usable with preference in the novel process for the suspension are one or more elements from the group of: Ru, Rh, Pd, Os, Ir and Pt. Noble metal oxides usable with preference in the novel process for the suspension are oxides of one or more elements from the group of: Ru, Rh, Pd, Os, Ir and Pt.

The valve metal compound used with preference, for example in the case of titanium as the electrode support, is a titanium alkoxide. Preference is given here to using titanium(IV) isopropoxide, titanium(IV) n-propoxide and titanium(IV) n-butoxide. It is also possible to add finely divided non-noble metal oxide powder, which may also be doped, to the coating solution or suspension. It is additionally possible in step a) to add non-noble metal and/or doping element precursor compounds which, on completion of drying and/or sintering, form a non-noble metal oxide which may also be doped, in which case the proportion of doping elements is preferably up to 20 mol % based on the total content of metals in the coating solution or dispersion.

Suitable non-noble metal precursor compounds are especially solvent-soluble fluorides, chlorides, iodides, bromides, nitrates, phosphates, acetates, acetylacetonates and/or alkoxides of the elements antimony, lead, iron, germanium, indium, manganese, molybdenum, niobium, tantalum, titanium, tellurium, vanadium, zinc, tin, and/or zirconium.

Preferred non-noble metal compounds are, alone or in a mixture:

titanium compounds selected from the group of: titanium fluoride, titanium chloride, titanium iodide, titanium bromide, titanium 2-ethylhexyloxide, titanium ethoxide, titanium isobutoxide, titanium isopropoxide, titanium methoxide, titanium n-butoxide, titanium n-propoxide, titanium tert-butoxide, and/or manganese compounds selected from the group of: manganese fluoride, manganese chloride, manganese iodide, manganese bromide, manganese nitrate, manganese phosphate, manganese acetate, manganese acetylacetonate, manganese methoxide, manganese ethoxide, manganese propoxide, manganese butoxide, and/or indium compounds selected from the group of: indium fluoride, indium chloride, indium iodide, indium bromide, indium nitrate, indium phosphate, indium acetate, indium acetylacetonate, indium ethoxide, indium propoxide, indium butoxide,and/or tin compounds selected from the group of: tin fluoride, tin chloride, tin iodide, tin bromide, tin acetate, tin acetylacetonate, tin methoxide, tin ethoxide, tin propoxide, tin butoxide, and/or zinc compounds selected from the group of: zinc fluoride, zinc chloride, zinc iodide, zinc bromide, zinc nitrate, zinc phosphate, zinc acetate, zinc acetylacetonate.

Particularly preferred non-noble metal-precursor compounds are tin chloride and/or indium chloride and/or manganese chloride.

The aforementioned compounds can also serve as a precursor compound for doping together with other compounds in the dispersion/solution.

Suitable doping element precursor compounds are especially solvent-soluble fluorine compounds and/or fluorides, chlorides, iodides, bromides, nitrates, phosphates, sulphates, acetates, acetylacetonates and/or alkoxides of the elements aluminium, antimony, tantalum, niobium, tin, indium, manganese. Preferred doping element precursor compounds are one or more compounds from the group of: manganese compounds selected from the group of manganese fluoride, manganese chloride, manganese iodide, manganese bromide, manganese nitrate, manganese phosphate, manganese acetate, manganese acetylacetonate, manganese methoxide, manganese ethoxide, manganese propoxide, manganese butoxide, and/or aluminium compounds selected from the group of: aluminium fluoride, aluminium chloride, aluminium iodide, aluminium bromide, aluminium nitrate, aluminium phosphate, aluminium sulphate, aluminium acetate, aluminium acetylacetonate, aluminium ethoxide, aluminium propoxide, aluminium butoxide and/or antimony compounds selected from the group of: antimony fluoride, antimony chloride, antimony iodide, antimony bromide, antimony sulphate, antimony acetate, antimony acetylacetonate, antimony methoxide, antimony ethoxide, antimony propoxide, antimony butoxide and/or tantalum compounds selected from the group of: tantalum fluoride, tantalum chloride, tantalum iodide, tantalum bromide, tantalum methoxide, tantalum ethoxides and/or niobium compounds selected from the group of: niobium fluoride, niobium chloride, niobium iodide, niobium ethoxide, niobium propoxide, niobium butoxide and/or indium compounds selected from the group of: indium fluoride, indium chloride, indium iodide, indium bromide, indium nitrate, indium phosphate, indium sulphate, indium acetate, indium acetylacetonate, indium ethoxide, indium propoxide, indium butoxide and/or tin compounds selected from the group of: tin fluoride, tin chloride, tin iodide, tin bromide, tin sulphate, tin acetate, tin acetylacetonate, tin methoxide, tin ethoxide, tin propoxide, tin butoxide.

Doping element precursor compounds suitable with particular preference are antimony chloride and/or tin chloride and/or manganese chloride.

It is known in principle from the prior art that the additional use of fluorine compounds as a doping additive can lead to an increase in the electrical conductivity of oxide particles. Doping of this kind is possible here especially when fluorine compounds selected from the group of: fluoroacetic acid, difluoroacetic acid, trifluoroacetic acid, hydrogen fluoride, ammonium fluoride, tetramethylammonium fluoride are additionally added to the solution/dispersion in step a). Preference is also given to an execution of the novel process, characterized in that the coating solution or dispersion according to step a) additionally comprises a precursor compound selected from the group of the compounds of: indium, tin, antimony, niobium, tantalum, fluorine and manganese, preferably tin, indium and antimony, as precursors for a doping, in which case the proportion of doping elements is preferably up to 20 mol % based on the total content of metals in the coating solution or dispersion. Very particular preference is given to the combination of tin compound as precursor compound with an antimony compound as dopant precursor or a combination of indium compound as precursor compound with a tin compound as dopant precursor. Thus, it is possible to obtain, in the end product, indium tin oxides (ITO) or tin antimony oxides (ATO) having a higher electrical conductivity than respectively undoped indium oxide or tin oxide

Preference is further given to a process in which the coating solution or dispersion according to step a) additionally comprises a precursor compound selected from the group of: tin(IV) chloride (SnCl₄), antimony(III) chloride (SbCl₃), indium(III) chloride (InCl₃) and manganese(II) chloride (MnCl₂).

By varying the composition of the coating solution or dispersion, it is possible to match the viscosity thereof to the respective application operation. It is possible to use, for example, the following substances known in principle: thickeners, stabilizers, wetting additives, etc. Preferred examples thereof are nonionic, water-soluble polyethylene oxide polymers and/or water-soluble methylcellulose and/or hydroxypropyl methylcellulose polymers, stabilizers such as polyvinyl alcohol and/or polyacrylic acid and/or polyvinylpyrrolidone, and wetting additives, for example anionic surfactants such as sodium dodecylsulphate or cationic surfactants such as dodecyltrimethylammonium chloride or nonionic surfactants such as polyethylene glycol monolaurate.

The coating solution or dispersion prepared is preferably applied to the surface of the support in two or more cycles. This can be done by brushing, spraying, flowing or dipping the carrier onto or into the coating solution or dispersion. Other application processes are likewise conceivable, for example printing processes.

The amount of noble metal or noble metal-containing coating which is applied to the electrode surface can be adjusted through the concentration of the application solution or through the number of repeat cycles. Electrodes for chlorine production and other gas-evolving reactions usually consist of expanded metals or other open structures. The figure for the noble metal loading per unit electrode area is based hereinafter on the geometric area of the electrode surface projected onto a plane to the normal. The amount of noble metal or noble metal-containing coating is then based on a surface area which can be calculated from the external dimensions of the electrode area.

After one application cycle, the liquid components of the coating solution or dispersion are removed by drying. Thereafter, it is possible to commence a new application step b) or to subject the electrode, after the drying c), to a sintering operation d) at a temperature of at least 300° C. Thereafter, the coating solution or dispersion can again be applied, dried and sintered.

The drying c) of the coating can be effected at standard pressure or else under reduced pressure, under air or optionally preferably under a mixture of oxygen with a protective gas, especially at least one gas from the group of nitrogen and noble gas, especially helium, neon, argon or krypton. This likewise applies to the sintering d) of the coating.

It is likewise conceivable to alter the formulation of the coating solution or dispersion in the different application and drying cycles, and thus to generate gradients in the coating structure. For instance, it is possible with preference first to coat with a low noble metal content of the supports and to increase the noble metal content on the carrier in further coating cycles. The supports used for the catalyst and coated may especially be sheetlike structures with complex geometry, for example expanded metals, perforated sheets, foams, knits, meshes and nonwovens.

Preference is additionally given to a process which is characterized in that the support is based on a valve metal from the group of titanium, zirconium, tungsten, tantalum and niobium, preferably titanium or alloys thereof or tantalum, more preferably on titanium or titanium alloys. Suitable titanium alloys, which are preferably more corrosion-stable than pure titanium, comprise, for example, palladium, or nickel and palladium and ruthenium and chromium, or nickel and molybdenum, or aluminium and niobium. In the case of a titanium alloy, preference is given to titanium-palladium (0.2% by weight Pd) and/or titanium-nickel-chromium-ruthenium-palladium (0.35 to 0.55% by weight of Ni, 0.1 to 0.2% by weight of Cr, 0.02 to 0.04% by weight of Ru and 0.01 to 0.02% by weight of Pd).

In a particularly advantageous variant of the novel process, the surface of the support before the application b) of the coating solution or dispersion to the support is mechanically pre-cleaned, especially sand-blasted and optionally subsequently etched with an acid such as hydrochloric acid or oxalic acid for further removal of oxides on the surface.

A particularly preferred process is characterized in that the support is based on metallic titanium or tantalum, preferably on titanium.

For production of a catalyst coating with binary mixed oxides formed from titanium oxide and ruthenium oxide, in a particularly preferred process, titanium(IV) n-butoxide (Ti[OCH₂CH₂CH₂CH₃]₄), ruthenium(III) chloride (RuCl₃), acetic acid (CH₃CO₂H), and water are used as reactants.

Another preferred variant of the novel process is characterized in that the sintering d) of the coated support is performed at a temperature of 300° C. to 700° C., preferably of 400° C. to 600° C. and more preferably of 450° C. to 550° C.

Through the alternative addition of further metal salts as dopants, for example iridium(III) chloride (IrCl₃), tin(IV) chloride (SnCl₄), antimony(III) chloride (SbCl₃) and manganese(II) chloride (MnCl₂), to the coating solution in the novel process, it is also possible with preference to obtain multinary mixed oxides.

The invention also provides a novel electrode with a novel catalyst coating, which is obtained as described above from the novel process.

The invention further provides for the use of the novel electrode for electrochemical production of chlorine from alkali metal chloride solutions or hydrogen chloride or hydrochloric acid, especially from sodium chloride solutions.

The novel electrodes which are obtained from the novel process may, as well as the described applications in chlorine production, alternatively also be used for generation of electrical power in, for example, fuel cells and batteries, in redox capacitors, in the electrolysis of water, in the regeneration of chromium baths, and in the case of use of fluorine-containing electrolytes in hydrogen peroxide, ozone of the peroxodisulphate production. These uses form a further part of the subject-matter of the invention.

The invention is illustrated in detail hereinafter by the examples, but these do not constitute a restriction of the invention.

EXAMPLES Example 1 Comparative Example with Commercial Anodes for NaCl Electrolysis and Electrolysis Test

A titanium-standard expanded metal anode from De Nora for NaCl electrolysis, which had been equipped with a ruthenium oxide- and iridium oxide-containing catalyst coating, was used. The size of the anode in the laboratory cell used was 10 cm×10 cm; the anode support material contained titanium and had the form of an expanded metal, characterized by the mesh size of 8 mm, land width 2 mm and land thickness 2 mm. Between the anode space and cathode space, a DuPont Nafion 982 ion exchange membrane was used. The cathode used was a standard nickel cathode from Denora for NaCl electrolysis, which had been equipped with a ruthenium-containing coating. The electrode separation was 3 mm. Introduced into the anode space of the electrolysis cell was an NaCl-containing solution having a sodium chloride concentration of 210 g/l, a volume flow rate between 5 and 10 l/h and a temperature of 88° C. On the cathode side of the cell, a sodium hydroxide solution having an NaOH concentration of 31.5% by weight, a volume flow rate between 5 and 10 l/h and a temperature of 88° C. was introduced. The current density was 4 kA/m², calculated based on the membrane area of 10 cm×10 cm. The chlorine concentration of the gas escaping from the anode chamber was 97% by volume. The electrolysis voltage was 3.05 V.

Example 2 Comparative Example

A coating solution comprising 2.00 g of ruthenium(III) chloride hydrate (Ru content 40.5% by weight), 9.95 g of n-butanol, 0.94 g of conc. hydrochloric acid (37% by weight), 5.93 g of tetra-n-butyl titanate (Ti—(O-Bu)₄) is made up. This is applied by brush to a sand-blasted expanded titanium metal with the same geometry as in Example 1 as a support. In the coating solution, the concentration of mineral acid is 27.3 mol % and the acid concentration thus totals 27.3 mol %. The ratio of the sum of the amounts of acid to the sum of the amounts of metal in the coating solution is 0.375.

Subsequently, the expanded metal is dried at 80° C. for 10 min and then sintered at 470° C. for 10 min. The application operation is repeated three times more, as are the drying and sintering. The last sintering operation is effected at 520° C. for 60 min. The areal ruthenium loading was determined from the consumption of the coating solution to be 20.8 g/m², with a composition of 31.5 mol % of RuO₂ and 68.5 mol % of TiO₂. This anode thus treated was used in a cell as in Example 1 in sodium chloride electrolysis with a commercial standard cathode. The chlorine concentration of the gas escaping from the anode chamber was 97.6% by volume. The electrolysis voltage was 3.06 V.

Example 3 Inventive Coating Production

Titanium sheets having a diameter of 15 mm (thickness 2 mm) were sand-blasted to clean and to roughen the surface and then etched in 10% oxalic acid at 80° C. (30 min), then cleaned with isopropanol.

Production of the coating solution: 4.00 g of titanium(IV) isopropoxide are added dropwise to an initial charge of 3.18 g of acetic acid while cooling in an ice bath with vigorous stirring, with at least 1 min of stirring time between two drops. The clear solution formed is then stirred while cooling for a further 12 h. To this is added dropwise a solution of 1.50 g of ruthenium(III) chloride hydrate (Ru content 40.35% by weight) in 29.74 g of 10% hydrochloric acid while cooling in an ice bath and stirring vigorously. Thereafter, the product is stirred for a further 96 h. In the coating solution prepared as described above, the concentration of organic acid is 34.3 mol %, the concentration of mineral acid 52.8 mol %, and the total acid concentration 87.0 mol %. The sum of the concentration of the metals is 13 mol %. The ratio of the sum of the amounts of acid to the sum of the amounts of metal in the coating solution is 6.71.

The coating solution was four times applied dropwise to titanium platelets or brushed onto expanded titanium metal. The areal ruthenium loading was determined from the increase in weight of the platelet to be 13.01 g/m², with a composition of 29.85 mol % of RuO₂ and 70.15 mol % of TiO₂.

Each application of the solution was followed by drying dry at 80° C. for 10 min and then sintering at 470° C. (under air) for 10 min. Application of the last layer was followed by sintering at 520° C. for another hour.

Example 4 Inventive Coating Production

Titanium sheets having a diameter of 15 mm (thickness 2 mm) were sand-blasted to clean and to roughen the surface and then etched in 10% oxalic acid at 80° C. (30 min), then cleaned with isopropanol.

Production of the coating solution: 4.78 g of tetra-n-butyl titanate are added dropwise to an initial charge of 12.85 g of acetic acid while cooling in an ice bath with vigorous stirring, with at least 1 min of stirring time between two drops. The clear solution formed is then stirred while cooling for a further 12 h. To this is added dropwise a solution of 1.68 g of ruthenium(III) chloride hydrate (Ru content 40.35% by weight), 0.544 g of tin(IV) chloride pentahydrate and 0.020 g of antimony(III) chloride in 26.0 g of 0.6% hydrochloric acid while cooling in an ice bath and stirring vigorously. Thereafter, the product is stirred for a further 96 h. In the coating solution prepared as described above, the concentration of organic acid is 88.9 mol %, the concentration of mineral acid 1.8 mol %, and the total acid concentration 90.7 mol %. The ratio of the sum of the amounts of acid to the sum of the amounts of metal in the coating solution is 9.75.

The coating solution was four times applied dropwise to titanium platelets or brushed onto expanded titanium metal. The areal ruthenium loading was determined from the increase in weight of the platelet to be 15.47 g/m², with a composition of 30.01 mol % of RuO₂, 62.85 mol % of TiO₂, 6.94 mol % of SnO₂ and 0.20 mol % of Sb₂O₅.

Each application of the solution was followed by drying dry at 80° C. for 10 min and then sintering at 470° C. (under air) for 10 min. Application of the last layer was followed by sintering at 520° C. for another hour.

Example 5 Inventive Coating Production

Titanium sheets having a diameter of 15 mm (thickness 2 mm) were sand-blasted to clean and to roughen the surface and then etched in 10% oxalic acid at 80° C. (30 min), then cleaned with isopropanol. Expanded titanium metal was sand-blasted to clean and to roughen the surface and then cleaned with isopropanol.

Production of the coating solution: To an initial charge of 14.35 g of acetic acid are added dropwise 4.78 g of tetra-n-butyl titanate while cooling in an ice bath with vigorous stirring, with at least 1 min of stirring time between two drops. The clear solution formed is then stirred while cooling for a further 12 h. To this is added dropwise a solution of 1.50 g of ruthenium(III) chloride hydrate (Ru content 40.35% by weight) in 17.84 g of demineralized water while cooling in an ice bath and stirring vigorously. Thereafter, the product is stirred for a further 96 h. In the coating solution prepared as described above, the concentration of acid is 92.3 mol %. The ratio of the sum of the amounts of acid to the sum of the amounts of metal in the coating solution is 11.93.

The coating solution was four times applied dropwise to titanium platelets or, in parallel, brushed onto expanded titanium metal. The areal ruthenium loading was determined from the increase in weight of the platelet to be 15.47 g/m², with a composition of 29.9 mol % of RuO₂, 70.1 mol % of TiO₂.

Each application of the solution was followed by drying at 80° C. for 10 min and then sintering at 470° C. (under air) for 10 min. Application of the last layer was followed by sintering at 520° C. for another hour.

Example 6 Inventive Coating Production

(Combination of production of fine particle addition and acid excess):

Titanium sheets having a diameter of 15 mm (thickness 2 mm) were sand-blasted to clean and to roughen the surface and then etched in 10% oxalic acid at 80° C. (30 min), then cleaned with isopropanol.

Production of the Coating Solution:

Solution 1: 1.59 g of tetra-n-butyl titanate were added dropwise to an initial charge of 4.77 g of acetic acid while cooling in an ice bath with vigorous stirring, with at least 1 min of stirring time between two drops. The clear solution formed was then stirred while cooling for a further 12 h.

Solution 2: 0.96 g of crenox TR-HP-2 from crenox GmbH (BET surface area 5-7 m²/g; density 4.2 g/cm³; TiO2-rutile 99.5% purity, density 4.2 g/cm³, mean particle size 206 nm (determined from electron micrograph as the average from 38 particles) was dispersed by means of ultrasound in a solution of 0.52 g of ruthenium(III) chloride hydrate (Ru content 40.35% by weight), 216 mg of tin(IV) chloride pentahydrate and 9 mg of antimony(III) chloride in 26.11 g of demineralized water for 1 h.

Solution 2 was added dropwise to solution 1, in the course of which solution 1 was cooled in an ice bath and stirred vigorously. Thereafter, the product was stirred for a further 96 h.

In the coating dispersion thus produced, the concentration of organic acid is 80.2 mol %, and the sum of the concentration of the metal compounds is 19.8 mol %. The ratio of the sum of the amount of acid to the sum of the amounts of metal in the coating solution is 4.05.

The coating solution was added dropwise to titanium platelets four times. The areal ruthenium loading was determined from the increase in weight of the platelets to be 4.85 g/m², with a composition of 86.0 mol % of TiO₂, 10.7 mol % of RuO₂, 3.2 mol % of SnO₂ and 0.1 mol % of Sb₂O₅.

Each application of the solution was followed by drying dry at 80° C. for 10 min and then sintering at 470° C. (under air) for 10 min. Application of the last layer was followed by sintering at 520° C. for another hour.

Example 7 Inventive Coating Production

Titanium sheets having a diameter of 15 mm (thickness 2 mm) were sand-blasted to clean and to roughen the surface and then etched in 10% oxalic acid at 80° C. (30 min), then cleaned with isopropanol.

Production of the coating solution: 4.00 g of titanium(IV) isopropoxide are added dropwise to an initial charge of 3.20 g of acetic acid while cooling in an ice bath with vigorous stirring, with at least 1 min of stirring time between two drops. The clear solution formed is then stirred while cooling for a further 12 h. To this is added dropwise a solution of 1.50 g of ruthenium(III) chloride hydrate (Ru content 40.35% by weight) in 29.78 g of 1% hydrochloric acid while cooling in an ice bath and stirring vigorously. Thereafter, the product is stirred for a further 96 h. In the coating solution prepared as described above, the concentration of organic acid is 65.4 mol %, the concentration of mineral acid 10.0 mol %, and the total acid concentration 75.4 mol %. The sum of the concentration of the metals is 24.6 mol %. The ratio of the sum of the amounts of acid to the sum of the amounts of metal in the coating solution is 3.06.

The coating solution was four times applied dropwise to titanium platelets or brushed onto expanded titanium metal. The areal ruthenium loading was determined from the increase in weight of the platelet to be 13.01 g/m², with a composition of 29.85 mol % of RuO₂ and 70.15 mol % of TiO₂.

Each application of the solution was followed by drying dry at 80° C. for 10 min and then sintering at 470° C. (under air) for 10 min. Application of the last layer was followed by sintering at 520° C. for another hour.

Example 8 Electrochemical Test on Platelets)

Electrochemical activity for chlorine evolution was measured on a laboratory scale for samples from Examples 3 to 7 on titanium electrodes (diameter 15 mm, thickness 2 mm) by recording polarization curves.

Test parameters: measured in 200 g/l NaCl (pH=3) at flow rate of 100 ml/min at 80° C., galvanostatic with 5 min per current stage, potential measured against Ag/AgCl and converted for standard hydrogen electrode (SHE), potential values were corrected to take account of ohmic potential drops in the cell (called IR correction), counterelectrode: platinized expanded titanium metal, current density 4 kA/m².

The potential, determined in each case at a current density of 4 kA/m², of the sample from Example 3 was 1.415 V; the sample from Example 4: 1.370 V; the sample from Example 5: 1.412 V; the sample from Example 6: 1.388 V and the sample from Example 7: 1.364 V.

Example 9

Laboratory Electrolysis with an Inventive Coating

The coating on expanded metal from Example 5 was used in a cell as in Example 1 in sodium chloride electrolysis with a commercial standard cathode. The electrolysis voltage after 3 days was 2.96 V, then the current density was increased to 6 kA/m² and the NaCl concentration was lowered to 180 g/l. After being 3.26 V initially, the electrolysis voltage fell somewhat, to 3.23 V after 10 days at 6 kA/m². The mean electrolysis voltage over the next 92 days was then 3.21 V. The chlorine concentration of the gas escaping from the anode chamber was 98.0% by volume after 3 days at 6 kA/m², and 97.0% after 102 days at 6 kA/m². Subsequently, the system was set back to the current density of 4 kA/m² at a brine concentration of 210 g/l and the electrolysis voltage was 2.92 V. 

1. A process for wet-chemical production of a catalyst coating on an electrically conductive support for electrodes for chloralkali or hydrochloric acid electrolysis with electrocatalytically active components based on at least one noble metal oxide and/or noble metal of the noble metals of transition group VIIIa of the Periodic Table of the Elements, and optionally additionally at least one valve metal oxide, especially titanium oxide, characterized in that the catalyst coating is produced by a) producing a coating solution or dispersion at least comprising a precursor compound of a noble metal and/or a metal oxide of a noble metal, especially of ruthenium and/or iridium, and optionally additionally a tin and/or valve metal compound, preferably a titanium compound, and a solvent or dispersant, with addition of one or more acids to the coating solution or dispersion, where the molar ratio of the total of the amounts of acid (in mol) present in the coating solution or dispersion to the sum of the amounts of the metals from the metal-containing components present in the coating solution or dispersion is at least 2:1, b) applying the coating solution or dispersion to the conductive support, c) substantially freeing the layer applied of solvent or dispersant by drying and d) then subjecting the dried layer obtained to a thermal treatment at a temperature of at least 300° C., and optionally in the presence of oxygen-containing gases, to form the catalyst coating.
 2. The process according to claim 1 wherein the coating solution or dispersion according to step a) comprises one or more acids having a pK_(a) in aqueous solution of not more than
 12. 3. The process according to claim 1 wherein the coating solution or dispersion according to step a) additionally comprises one or more precursor compounds and/or a metal and/or a metal oxide of one or more doping elements selected from the group consisting of: aluminium, antimony, lead, iron, germanium, indium, manganese, molybdenum, niobium, tantalum, titanium, tellurium, vanadium, zinc, tin and zirconium.
 4. The process according to claim 1 wherein the thermal treatment d) is performed in the presence of air or in the presence of a mixture of oxygen and a protective gas selected from the group consisting of nitrogen, helium, neon, argon or krypton.
 5. The process according to claim 1 wherein the drying c) is performed in the presence of air or in the presence of a mixture of oxygen and a protective gas, especially at least one protective gas selected from the group consisting of nitrogen, helium, neon, argon and krypton.
 6. The process according to claim 1 wherein the precursor compound used in step a) is a chloride of ruthenium and/or iridium.
 7. The process according to claim 1 wherein the acid used in the coating solution in a) is an organic or inorganic acid or a combination of organic and inorganic acids.
 8. The process according to claim 7, wherein the acid used in the coating solution is a combination of organic and inorganic acid, where the molar ratio of the amount of organic acid (in mol) present in the coating solution or dispersion to the amount of mineral acid is 20:80 to 100:0.
 9. The process according to claim 7 wherein the acid used in the coating solution in a) is hydrochloric acid and/or a C₁ to C₄-carboxylic acid selected from the group consisting of: hydrochloric acid, formic acid, acetic acid and propionic acid.
 10. The process according to claim 1 wherein the solvent or dispersant used is one or more from the group of: water and C₁-C₆-alcohol, preferably butanol or isopropanol.
 11. The process according to claim 1 wherein the support is based on a valve metal selected from the group consisting of titanium, zirconium, tungsten, tantalum and niobium.
 12. The process according to claim 1 wherein the surface of the support before the application b) of the coating solution or dispersion to the support is mechanically pre-cleaned and optionally subsequently etched with an acid such as hydrochloric acid or oxalic acid for further removal of oxides on the surface.
 13. The process according to claim 1 wherein the coating solution or dispersion according to step a) additionally comprises at least one precursor compound selected from the group of the compounds of: aluminium, antimony, lead, iron, germanium, indium, manganese, molybdenum, niobium, tantalum, titanium, tellurium, vanadium, zinc, tin, and zirconium, tin, indium, manganese and antimony, optionally also as a precursor compound for a doping, in which case the proportion of doping elements is ultimately preferably up to 20 mol % based on the total content of metals in the coating solution or dispersion.
 14. The process according to claim 13, wherein the coating solution or dispersion according to step a) comprises one or more precursor compounds selected from the group of the compounds of: aluminium, antimony, tantalum, niobium, tin, indium, more preferably from the group of tin(IV) chloride (SnCl₄), indium(III) chloride (InCl₃), antimony(III) chloride (SbCl₃) and manganese(II) chloride (MnCl₂).
 15. The process according to claim 1 wherein the additional valve metal precursor compounds used are solvent-soluble fluorides, chlorides, iodides, bromides, nitrates, phosphates, sulphates, acetates, acetylacetonates or alkoxides of the elements titanium, zirconium, tungsten, tantalum and niobium.
 16. The process according to claim 1 wherein a catalyst coating is produced with binary mixed oxides formed from titanium oxide and ruthenium oxide by using a coating solution in a) comprising titanium(IV) butoxide and ruthenium(III) chloride (RuCl₃) and a mixture of acetic acid and water.
 17. An electrode having a catalyst coating obtained by a process according to claim
 1. 18. A method for using the electrode according to claim 17 for electrochemical production of chlorine from hydrogen chloride or alkali metal chloride solutions, especially from sodium chloride solutions, or for generation of electrical power, preferably in fuel cells and batteries, in redox capacitors, for electrolysis of water, for regeneration of chromium baths, or in the case of use of fluoride-containing electrolytes in hydrogen peroxide, ozone of the peroxodisulphate production. 